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Mixed-dimensional perovskites containing mixtures of organic cations hold great promise to deliver highly stable and efficient solar cells. However, although a plethora of relatively bulky organic cations have been reported for such purposes, a fundamental understanding of the materials’ structure, composition, and phase, along with their correlated effects on the corresponding optoelectronic properties and degradation mechanism remains elusive. Herein, we systematically engineer the structures of bulky organic cations to template low-dimensional perovskites with contrasting inorganic framework dimensionality, connectivity, and coordination deformation. By combining X-ray single-crystal structural analysis with depth-profiling XPS, solid-state NMR, and femtosecond transient absorption, it is revealed that not all low dimensional species work equally well as dopants. Instead, it was found that inorganic architectures with lesser structural distortion tend to yield less disordered energetic and defect landscapes in the resulting mixed-dimensional perovskites, augmented in materials with a longer photoluminescence (PL) lifetime, higher PL quantum yield (up to 11%), improved solar cell performance and enhanced thermal stability (T80 up to 1000 h, unencapsulated). Our study highlights the importance of designing templating organic cations that yield low-dimensional materials with much less structural distortion profiles to be used as additives in stable and efficient perovskite solar cells. Ministry of Education (MOE) National Research Foundation (NRF) Published version The authors would like to acknowledge funding from the Singapore National Research Foundation through the IntraCREATE Collaborative Grant (NRF2018-ITC001-001), Energy Innovation Research Program (NRF2015EWT-EIRP003-004 and Solar CRP: S18-1176-SCRP), and MOE Tier 2 project MOE2019-T2-2-097.

Extending halide perovskites' optoelectronic properties to stimuli-responsive chromism enables switchable optoelectronics, information display, and smart window applications. Here, we demonstrate a band gap tunability (chromism) via crystal structure transformation from three-dimensional FAPbBr3 to a ⟨110⟩ oriented FAn+2PbnBr3n+2 structure using a mono-halide/cation composition (FA/Pb) tuning. Furthermore, we illustrate reversible photochromism in halide perovskite by modulating the intermediate n phase in the FAn+2PbnBr3n+2 structure, enabling greater control of the optical band gap and luminescence of a ⟨110⟩ oriented mono-halide/cation perovskite. Proton transfer reaction-mass spectroscopy carried out to precisely quantify the decomposition product reveals that the organic solvent in the film is a key contributor to the structural transformation and, therefore, the chromism in the ⟨110⟩ structure. These intermediate n phases (2 ≤ n ≤ ∞) stabilize in metastable states in the FAn+2PbnBr3n+2 system, which is accessible via strain or optical or thermal input. The structure reversibility in the ⟨110⟩ perovskite allowed us to demonstrate a class of photochromic sensors capable of self-adaptation to lighting. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This research was funded by National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award No. NRF-CRP14-2014-03) and Ministry of Education, Singapore (MOE2019-T2-2-097).

Zero-dimensional (0D) hybrid organic–inorganic lead halides have been shown to display efficient broadband photoluminescence and are, therefore, of significant interest for artificial lighting applications. However, work that investigates the formability of the materials as a function of templating organic cation structure is rare. This severely limits our ability to rationally design new materials displaying specific structural and photophysical properties. With the goal of accessing rare 0D trimeric bromoplumbates, we have systematically varied templating N-alkylpyridinium cations and examined their impact upon inorganic lattice structure. Whereas comparatively short and flexible N-alkyl substituents (ethyl, 2-hydroxyethyl, and pentyl) yield one-dimensional (1D) inorganic chains, more rigid substituents (benzyl, acetamidyl, and cyanomethyl) afford hybrids composed of lead bromide face-sharing trimers (i.e., [Pb3Br12]6–). Of the rigid substituents studied, benzyl groups were found to enforce the highest level of distortion of the [PbBr6]4– octahedra that comprise their trimeric structures. Upon exposure to ultraviolet (UV) light, N-benzylpyridinium lead bromide (1)6[Pb3Br12] exhibits a broadband emission, centered at 571 nm, which spans from 400 to 800 nm. More specifically, it displays a large Stokes shift of ca. 1.39 eV and a full width at half-maximum of ca. 146 nm. This broadband emission decays with a comparatively long lifetime of 426 ns at room temperature, which increases to 5.8 μs at 77 K. The reduced size and dimensionality of its inorganic lattice also result in a photoluminescence quantum yield (of at least 10%) that is approximately one order of magnitude higher than that of its 1D congeners. Mechanistically, broadband emission in (1)6[Pb3Br12] is believed to originate from triplet excited state(s) obtained from excited-state structural reorganization of the [Pb3Br12]6– moiety. Accepted version

Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra-thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide-angle range. Here, a tunable perfect absorption from normal incidence to a wide-angle range (0° to 70°) by utilizing a two-layer stack consisting of a high refractive index low-loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low-loss phase change material such as stibnite (Sb2 S3 ) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb2 S3 from amorphous to crystalline. Agency for Science, Technology and Research (A*STAR) P.P. and R.S. acknowledge the funding support from Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) by Agency for Science, Technology, and Research (A*STAR). K.V.S. and J.T. acknowledge A*STAR for funding support in Grant Nos. H19H6a0025, A20E5c0084, A2083c0058 and CRF SC25/21-110318.

Major strides have been made in the development of materials and devices based around low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work has mostly focused on compounds containing highly toxic Pb, with the analogous less toxic Sn materials being comparatively poorly evolved. In response, the study herein aims to (i) provide insight into the impact of templating cations upon the structure of

Major strides have been made in the development of materials and devices based around low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work has mostly focused on compounds containing highly toxic Pb, with the analogous less toxic Sn materials being comparatively poorly evolved. In response, the study herein aims to (i) provide insight into the impact of templating cations upon the structure of n = 1 2D tin iodide perovskites (where n refers to the number of contiguous two-dimensional (2D) inorganic layers, i.e., not separated by organic cations) and (ii) examine their potential as light absorbers for photovoltaic (PV) cells. It was discovered through systematic tuning of organic dications that imidazolium rings are able to induce the formation of (110)-oriented materials, including examples of "3 × 3" corrugated Sn-I perovskites. This structural outcome is a consequence of a combination of supramolecular interactions of the two endocyclic N atoms of the imidazolium rings with the Sn-I framework, and the comparatively high tendency of Sn2+ ions to stereochemically express their 5s2 lone pairs . More importantly, the resulting materials feature very short separations between their 2D inorganic layers with iodide-iodide (I···I) contacts as small as 4.174 Å, which is among the shortest ever recorded for 2D tin iodide perovskites. These proximate inorganic distances, combined with the polarizable nature of the imidazolium moiety, eases the separation of photogenerated charge within the materials. This is evident from the measurement of excitonic activation energies as low as 83(10) meV for ImEA[SnI4]. When combined with superior light absorption capabilities relative to their lead congeners, this allowed the fabrication of lead-free solar cells with incident photon-to-current and power conversion efficiencies of up to 70% and 2.26%, respectively, which are among the highest values reported for pure n = 1 2D group 14 metal halide perovskites. In fact, these values are superior to the corresponding lead iodide material, which demonstrates that 2D Sn-based materials have significant potential as less toxic alternatives to their Pb counterparts. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) J.E.is grateful to NTU for funding (M4081442). S.G. M., N.M., B.F., T. M.K., and T.J.N.H. would like to acknowledge funding from the Singapore National Research Foundation through the IntraCREATE Collaborative Program (NRF2018-ITC001-001), Office of Naval Research Global (ONRG-NICOP-N62909-17-1-2155), and the Competitive Research Program: NRF-CRP14-2014-03. Y.L. and Z.X.S. would like to acknowledge Ministry of Education of Singapore for the funding through AcRF Tier 1 (ref RG195/17 and RG156/19).

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light-emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier-phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two-step process leads to a unique pump- and probe-energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton-phonon coupling of 2D perovskites relevant for emergent optoelectronics development. Ministry of Education (MOE) National Research Foundation (NRF) This research was supported by Nanyang Technological University under its start-up grant (M4080514); the Ministry of Education under its AcRF Tier 1 grant (RG91/19) and Tier 2 grants (MOE2017-T2-2-002, MOE2019-T2-1-006 and MOE2019-T2-1-097); and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04). Z.S. gratefully acknowledges Ministry of Education Singapore for the funding of this research through the following grants: AcRF Tier 1 (RG195/17 and RG156/19) and AcRF Tier 3 (MOE2016-T3-1-006 (S)).

Combining the unique properties of hybrid halide perovskites and in-plane anisotropic potential of two-dimensional materials, corrugated layered perovskites are excellent candidates for novel optoelectronic devices. Here, we report strong in-plane anisotropy in a two-dimensional hybrid halide perovskite with corrugated inorganic sheets. We assigned the angle-dependent Raman spectra to particular orientations of the crystals where the method applied is suitable for fast determination of the in-plane crystallographic axes. We demonstrated strong in-plane anisotropy of the optoelectronic properties: electrical measurements showed that changes of photoconductivity and angle-resolved reflectance spectroscopy exhibited variation of the band structure. Employing first-principles calculations, we confirmed the observed anisotropy of the dielectric response. Photoluminescence spectroscopy revealed an anomalous energy shift of μ20 meV dependent on the relative orientation of the crystals and polarization of the excitation light. Using temperature-dependent analysis of the photoluminescence spectra, we obtained the anisotropic exciton-phonon coupling strength as well as the angle-dependent average phonon energy involved, which contributes to the shift of the photoluminescence peak maximum. This discovery of the in-plane anisotropic behavior in layered 2D perovskites could be expected to spur new functionalities in optical and optoelectronic device applications. Ministry of Education (MOE) The authors gratefully acknowledge the Ministry of Education (MOE) of Singapore for the funding of this research through the following grants, AcRF Tier 1 (Reference No: RG195/17); AcRF Tier 1 (RG165/19); AcRF Tier 3 (MOE2016-T3-1-006 (S)). J.Y. acknowledges the National Natural Science Foundation of China (Grant No. 11704185), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20171021), and the funding for "High-level talents in six industries" of Jiangsu Province (Grant No. XCL-020).

Major strides have been made in the development of materials and devices based around low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work has mostly focused upon compounds containing highly toxic Pb, with the analogous less toxic Sn materials being comparatively poorly evolved. In response, the study herein aims to (i) provide insight into the impact of templating cation upon the structure of 2D tin iodide perovskites, and (ii) examine their potential as light absorbers for photovoltaic (PV) cells. It was discovered through systematic tuning of organic dications, that imidazolium rings are able to induce formation of (110)-oriented materials, including the first examples of “3 × 3” corrugated Sn-I perovskites. This structural outcome is a consequence of a combination of supramolecular interactions of the two endocyclic N-atoms in the imidazolium functionalities with the Sn-I framework and the higher tendency of Sn2+ ions to stereochemically express their 5s2 lone pairs relative to the 6s2 electrons of Pb2+. More importantly, the resulting materials feature very short separations between their 2D inorganic layers with iodide–iodide (I···I) contacts as small as 4.174 Å, which is amongst the shortest ever recorded for 2D tin iodide perovskites. The proximate inorganic distances, combined with the polarizable nature of the imidazolium moiety, eases the separation of photogenerated charge within the materials. This is evident from the excitonic activation energies as low as 83(10) meV, measured for ImEA[SnI4]. When combined with superior light absorption capabilities relative to their lead congeners, this allowed fabrication of lead-free solar cells with incident photon-to-current and power conversion efficiencies of up to 70 % and 2.26 %, respectively, which are amongst the highest values reported for pure 2D group 14 metal halide perovskites. In fact, these values are superior to the corresponding lead iodide material, which demonstrates that 2D Sn-based materials have significant potential as less toxic alternatives to their Pb counterparts.

We utilized two organic dications containing, respectively, a pyridinium and an imidazolium core to construct new n = 1 (where n refers to the number of contiguous two-dimensional (2D) inorganic layers, i.e., not separated by organic cations) 2D lead-iodide perovskites 1 and 2. The former material exhibits a (100)- and the latter a very rare 3 � 3 (110)-structural type. Compared with primary ammonium functionality, their constituent ring-centered positive charges have lower charge density. As a result, PbI 6 4- interoctahedral distortions of the inorganic lattice in 1 and 2 are reduced (Pb-I-Pb bond angles are as high as 166° and 174°, respectively). This results in bathochromically shifted optical features. In addition, the compact nature of the dications produces extremely short lead-iodide sheet separations, with respective iodide-iodide (I···I) distances as small as 4.149 and 4.278 à . These are among the shortest separations of adjacent lead-iodide layers ever reported for such materials. When crystallized as thin films on top of substrates, the resulting 2D perovskite layers do not adopt a regular growth direction parallel to the surface. Instead, the crystallites grow with no fixed orientation. As a consequence of their proximate inorganic distances and unusual crystallization tendencies, the resulting 2D perovskites exhibit low excitonic activation energies (93.59 and 96.53 meV, respectively), enhanced photoconductivity in solar cells, and unprecedented incident photon-to-current conversion rates of up to 60%. More importantly, mesoporous 2D layered perovskite solar cells with power conversion efficiencies of 1.43 and 1.83% were achieved for 1 and 2, respectively. These are the highest values obtained thus far for pure n = 1 lead-iodide perovskites and more than 20 times higher than those obtained for materials templated by more conventional cations such as phenylethylammonium (0.08%). Copyright © 2019 American Chemical Society.

Pure 2D lead‐iodide perovskites typically demonstrate poor charge transport and compromised visible light absorption, relative to their 3D congeners. This hinders their potential use as solar absorbers. Herein, the systematic tuning of pyridinium‐based templating cations is reported to introduce intermolecular interactions that provide access to a series of new 2D lead‐iodide perovskites with reduced inter‐octahedral distortions (largest Pb‐(μ‐I)‐Pb bond angles of 170–179°) and very short inorganic interlayer separations (shortest I⋅⋅⋅I contacts ≤4.278–4.447 Å). These features manifest in reduced band gaps (2.35–2.46 eV) and relaxed dielectric confinement (excitonic binding energies of 130–200 meV). As a consequence, they demonstrate (more than ten‐fold) improved photo‐ and electrical conductivities relative to conventional 2D lead‐iodide perovskites, such as that templated by 2‐(1‐naphthyl)ethylammonium. Through computational studies, the origin of this behavior was shown to derive from a combination of short iodoplumbate layer separations and the aromaticity of the organic dications. NRF (Natl Research Foundation, S’pore) Accepted version

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self‐assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton–photon coupling, etc. Detailed understanding into the strong light–matter interactions in these hybrid organic–inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin–quasiparticle interactions in these layered 2D halide perovskites with large spin–orbit coupling. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version